Watt-hour meter

Watt-hour meter

An electrical energy meter, that is, an electricity meter that measures and registers the integral, with respect to time, of the power in the circuit in which it is connected. This instrument can be considered as having two parts: a transducer, which converts the power into a mechanical or electrical signal, and a counter, which integrates and displays the value of the total energy that has passed through the meter. Either or both of these parts can be based on mechanical or electronic principles.

In its wholly mechanical form the transducer is an electric motor designed so that its torque is proportional to the electric power in the circuit. The motor spindle carries a conducting disk that rotates between the poles of one or more strong permanent magnets. These provide a braking torque that is proportional to the disk rotational speed, so the motor runs at a rate that accurately represents the circuit power. The integrating register is simply connected to the motor through a gear train that gives the required movement of the dials in relation to the passage of electrical energy. See Motor

Mechanical meters can measure either dc or ac energy. Some form of commutator motor similar to those with a shunt field winding is commonly used for dc energy measurement. It is most convenient for the field to carry the circuit current, while the armature is fed with a signal from the circuit voltage. The Ferraris, or induction-type, meter is used for ac energy measurement. The stator carries two windings. An ordinary energy meter will easily achieve an accuracy of 2% over a wide range of loads; precision models may reach 0.1%. Ferraris meters are in very wide use and measure the consumption of the vast majority of domestic and industrial users of electric power throughout the world.

Electronic meters have an electronic watt transducer, which is a solid-state circuit that performs the multiplication of current and voltage signals, and delivers an output in the form of a pulse train at a rate proportional to power. The simplest solid-state watt-hour meter is completed by adding an electronic register to record the energy consumed. Precision electronic energy meters can give errors less than 0.005%. Electronic instruments are available in which six registers are provided, to record consumption at four different times of day and two levels of maximum demand. See Transducer, Voltmeter

Intelligent, or smart, meters can provide a wide variety of load and tariff-control functions, as well as remote reading of energy consumption.

Electronic and mechanical techniques can be combined in a variety of ways. Signals from a mechanical transducer may be used to operate electronic registers in order to obtain the advantages of the facilities that they can provide. A mechanical impulse register may be used in conjunction with an electronic transducer, where it is considered important to maintain a record without the need for batteries or nonvolatile memory elements.

Watt-hour meters that operate at potentials of 100–250 V and currents up to 100 A are widely manufactured. At higher levels, voltage or current transformers are used to reduce the signals handled by the meter to more convenient values, frequently 110 V and 5 A. By this means it is possible to carry out energy metering at any level required, including hundreds of kilovolts and tens of kiloamperes. See Electric power measurement, Instrument transformer, Transformer